The present invention relates to an electric actuator, and more particularly, to an electric actuator that can be used in place of a fluid pressure cylinder that is a part of an automated machine.
In automated machines such as machine tools and packaging machines, pneumatic cylinders are widely used to move an object or to apply pressing force to an object to hold it, or example, against a wall surface. However, the efficiency of a compressor producing compressed air for driving a pneumatic cylinder is relatively poor. In terms of the total amount of used energy, the use of electricity as the drive source consumes less energy than the case where a pneumatic cylinder is used. Accordingly, electric actuators have been used, in which an electric motor is used in place of a pneumatic cylinder. Specifically, rotation of the electric motor is converted into a linear motion to cause an output shaft to reciprocate linearly.
In an electric actuator, rotation of a motor (output shaft) is detected with a rotary encoder, and the displacement and the moving velocity of the output shaft can be determined based on detection signals from the rotary encoder. It is thus possible to perform feedback control using detection signals of the rotary encoder. Compared to a pneumatic cylinder, the displacement and the velocity of the output shaft of an electric actuator are more flexibly controlled. An electric actuator is controlled by performing feedback control on the rotation amount and rotation speed of the motor. The control of an electric actuator is thus different from the control of a pneumatic cylinder, in which the displacement of a piston rod is controlled through ON/OFF control using an electromagnetic valve. Therefore, when using an electric actuator in place of a pneumatic cylinder, a control method that is different from that of a pneumatic cylinder is required.
Conventionally, a linear actuator has been known that includes a motor having a motor drive circuit controlling the motor power, an encoder for detecting the rotation position of the motor, a motor control circuit that performs closed-loop control on the motor based on commands from a host command unit, and a motion converting mechanism that converts rotation of the motor output shaft into linear motion. Prescribed operations of the motor include positioning of the linear actuator, control of the velocity of an end effecter, control of thrust of the end effecter, and combinations of the listed operations. In response to commands from a host command unit such as a personal computer (hereinafter, referred to as a PC) or a programmable logic controller (hereinafter, referred to as a PLC), the motor control circuit executes a single operation command or a command that has been taught and memorized in advance.
The linear actuator disclosed in Japanese Laid-Open Patent Publication No. 2000-92811 is capable of performing single operation commands or previously taught and memorized commands based on commands from a host command unit. In order to control a pneumatic cylinder, three types of electromagnetic valves, namely a two-position single solenoid valve, a two-position double solenoid valve, and a three-position double solenoid valve are selectively used in combination depending on the operation pattern. Even if a command signal from the host command unit is the same, the operation of the pneumatic cylinder differs depending on the type of the used electromagnetic valve.
For example, in the case of a two-position single solenoid valve, and there is one input channel from a host command unit, while an ON signal is supplied from the host command unit, the piston is moved away from a home position (reference position). When the supply of the ON signal is stopped, the piston is returned to the home position.
In the case of a two-position double solenoid valve or a three-position double solenoid valve, there are two input channels from a host command unit. In a two-position double solenoid valve, when a signal through the first input channel from the host command unit is turned ON with the piston at the home position, the piston is moved away from the home position (reference position), and this motion continues even if the signal through the first input channel is turned OFF. After the piston reaches a target position, when a signal through the second input channel is turned ON, the piston is moved toward the home position. This motion continues even if the signal through the second input is turned OFF. That is, the piston does not stop at any intermediate position.
In the case of a three-position double solenoid valve, when a signal through the first input channel from the host command unit is turned ON with the piston at the home position, the piston is moved away from the home position (reference position), and the piston is stopped when the signal through first input channel is turned OFF. When a signal through the second input channel from the host command unit is turned ON with the piston at a position away from the home position, the piston is moved toward the home position, and the piston is stopped when the signal through the second input channel is turned OFF. That is, the piston stops at an intermediate position.
Therefore, in order to control a linear actuator (electric actuator) using a control circuit designed to control a pneumatic cylinder without modification, programs need to be taught to the motor control circuit of the linear actuator in correspondence with each of these electromagnetic valves. The replacement is thus troublesome. Japanese Laid-Open Patent Publication No. 2000-92811 gives no consideration to the relationship with types of electromagnetic valves designed to control pneumatic cylinders.